The present invention relates to a method and apparatus for termination of a screen of a cable. In particular, the invention relates to an improved ferrule for terminating the screen of a signal carrying cable and a method of connection of a screened cable to an electrical connector.
Screened cables, for example screened twisted pairs (STP) and coaxial cables, commonly have their screen terminated by one or more crimped ferrules, In the case of STP cables the screen is commonly in the form of a foil or plating on the inside of a plastic sleeve. The foil screen commonly includes a drain wire running along the length of the cable in contact with the foil. Termination of a foil screen is commonly performed by stripping back both the foil and jacket of the cable, folding the exposed drain wire back over the jacket and crimping a ferrule around the jacket and the drain wire. The purpose of the screen is to shield the twisted wire pairs from external electrostatic or electromagnetic energy.
In an alternative termination commonly employed with coaxial cables, the cable is prepared prior to termination by stripping back a section of the cable's jacket to reveal a braided screen. A first cylindrical ferrule is slid onto the cable over the jacket and the screen is then wrapped back over the first ferrule. A second, slightly larger ferrule is then slid over the first ferrule and the screen is crimped into place. The crimping force secures the wrapped back screen between the two ferrules and also secures the first ferrule to the jacket. Signal carrying element(s) of the cable is/are then terminated to a connector, the body of which is connected to the ferrules to continue the shielding. In this case the screen continuity is primarily intended to maintain the characteristic impedance of the cable.
The screen can be a braiding that consists of a of a plurality of fine wires woven into a hollow cylinder disposed between the signal carrying element(s) of the cable and the jacket.
Both of the above described termination procedures result in a drop in the screening performance of the cable at the cable-connector interface. In the case of crimping the screen between the two ferrules, the crimp does not produce uniform engagement between the ferrules and the screen. In the case of the drain wire, while the cable is completely screened about its circumference by the foil, the screen-connector interface is formed by a single wire crimped to a ferrule. The connection is formed by only a few millimeters of contact between the wire and ferrule and screening performance is therefore greatly reduced. Furthermore, in both cases there is a gap in the screening at the cable connector interface allowing leakage at the terminated end of the cable.
An alternative applicant explored to overcome these problems involved wrapping the foil screen back over the jacket of the cable and crimping a connector body over the wrapped back foil. The foil is commonly in the form of a sheet wrapped around the signal carrying element(s), normally with a longitudinal seam. The sheet, when wrapped back over the jacket, is not sufficiently wide to completely encircle the cable and therefore results in a gap such that the screening performance in this region is still reduced. Furthermore, the foil commonly tears and kinks during wrapping back, therefore reducing screening effectiveness.
The present invention seeks to provide a screen terminating ferrule for connecting a screened cable to a connector, the connection between the screen, ferrule and connector being simple and having improved performance over those described. The present invention also seeks to provide a method for the termination of screened cables using the screen terminating ferrule.